The shear zone-related gold mineralization at the Turmalina deposit, Quadrilátero Ferrífero, Brazil: structural evolution and the two stages of mineralization

Abstract

Turmalina is an important orogenic gold deposit located in the NW region of the Quadrilátero Ferrífero. The deposit is hosted in an Archean greenstone belt composed of ortho-amphibolites and pelites with interleaved tuffs metamorphosed under amphibolite facies conditions and intruded by a granite stock. The orebodies are controlled by WNW-ESE-trending shear zones, associated with hydrothermal alteration. Three deformation events are recognized in the Turmalina gold deposit: D1 and D2 are the result of a progressive Archean deformation under ductile conditions between 2749 ± 7 and 2664 ± 35 Ma; D3 is characterized by a transpressional event under ductile-brittle conditions with the age still unclear. The three generations of garnet observed show that Grt1 blastesis is pre- to syn-D1 and Grt2 growth during the late to post-deformation stages of the D2 event. The initial temperature (Grt1 core) is around 548–600 °C, whereas during late D2, the temperatures reached 633 °C (metamorphic peak–Grt2 rim), likely as a result of granite intrusion. The Grt3 resulted from re-equilibration under retrograde conditions. Two gold-bearing sulfide stages were identified: pyrrhotite-arsenopyrite ± löllingite ± chalcopyrite ± gold stage I precipitated below a metamorphic peak temperature of 598 ± 19 °C associated with S1 foliation (D1), and pyrrhotite-pyrite-arsenopyrite ± chalcopyrite ± gold stage II is located commonly along V3 quartz-carbonate veinlets with a temperature range between 442 ± 9 and 510 ± 30 °C. We suggest that the granite intrusion imposed an additional thermal effect that promoted further dehydration of country rocks. The Au derived mainly from a metamorphic fluid source but potentially mixed with magmatic fluids from the granite.

Notes

Acknowledgements

The authors wish to acknowledge the Jaguar Mining Co. for their technical, logistic, and financial support during our research. Special thanks to Carlos Ribeiro Luiz and all technicians, helpers, and geologists in the Turmalina Mine: Williams Santos, Romulo Cruz, Carlos Michel, Alvania Augusta, Armando Filho, Lucas, and Aloma Tente. Numerous geologists and many people must be thanked for their support and discussions: Ana-Sophie Hensler, Fabrício Caxito, Atlas Corrêa-Neto, Haakon Fossen, Lydia Lobato, Jaqueline Menez, Ruy Vasconcelos, William Campos, and Maria Emília Della Giustina. We thank the Mineralium Deposita reviewers (Dr. Ridley and Dr. Micklethwaite) and the AE (Dr. Steffen Hagemann) for providing constructive and helpful comments. We also acknowledge the support of Brasilia University, UnB, which improved the conditions to do electron microprobe analyses and LA-ICP-MS.

Note: It is with a certain sense of sadness that we report the passing of Dr. Bernhard Bühn. Bernhard is a true scholar and a gentleman. He was a pioneer in the UnB’s geochronology laboratory, especially for the ICP-MS method, and left a great legacy.

Funding information

This research is financed by Brazil’s Personal Improvement Coordination of Higher Education, CAPES.